Circumference: This is the distance all the way around the circle. Imagine walking along the circle’s edge once and measuring how far you walked—that distance is the circumference.
Radius: The radius is a straight line from the centre of the circle to any point on its edge. It’s half the length of the circle’s widest distance.
Diameter: The diameter is a straight line that goes from one side of the circle to the other, passing through the centre. It’s twice the length of the radius.
Semicircle: A semicircle is half of a circle. If you cut a circle along its diameter, you end up with a shape that looks like a half-moon or half a pizza. This half-circle is called a semicircle.
Arc: An arc is part of the circle’s edge between two points. It’s like a curved line on the circle’s outline.
Sector: A sector is a “slice” of the circle, made by two radius lines and the arc between them. It looks like a piece of pizza or pie.
Chord: A chord is any straight line that connects two points on the circle’s edge without necessarily passing through the centre.
Segment: A segment is the area inside the circle between a chord (a straight line) and the arc above or below it. Imagine cutting a small “cap” off the circle with a straight line.
Tangent: A tangent is a straight line that just touches the circle at one single point on its edge. It doesn’t cut through the circle.
Secant: A secant is a straight line that passes through the circle, cutting it in two places. It enters one side of the circle and goes out the other.
If you think you know all of the names of te parts of a circle you can find more activities on the Circles topic page.
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